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                <ol class="chapter"><li class="chapter-item expanded "><a href="ch01-00-get-excited.html"><strong aria-hidden="true">1.</strong> Get excited</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="index.html"><strong aria-hidden="true">1.1.</strong> Check out the README</a></li><li class="chapter-item expanded "><a href="Demo.html"><strong aria-hidden="true">1.2.</strong> See the demo</a></li></ol></li><li class="chapter-item expanded "><a href="ch02-00-learn-to-use.html"><strong aria-hidden="true">2.</strong> Learn to use Watermelon</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="Installation.html"><strong aria-hidden="true">2.1.</strong> Installation</a></li><li class="chapter-item expanded "><a href="Setup.html"><strong aria-hidden="true">2.2.</strong> Setup</a></li><li class="chapter-item expanded "><a href="Schema.html" class="active"><strong aria-hidden="true">2.3.</strong> Schema</a></li><li class="chapter-item expanded "><a href="Model.html"><strong aria-hidden="true">2.4.</strong> Defining Models</a></li><li class="chapter-item expanded "><a href="CRUD.html"><strong aria-hidden="true">2.5.</strong> Create, Read, Update, Delete</a></li><li class="chapter-item expanded "><a href="Components.html"><strong aria-hidden="true">2.6.</strong> Connecting to React Components</a></li><li class="chapter-item expanded "><a href="Query.html"><strong aria-hidden="true">2.7.</strong> Querying</a></li><li class="chapter-item expanded "><a href="Relation.html"><strong aria-hidden="true">2.8.</strong> Relations</a></li><li class="chapter-item expanded "><a href="Writers.html"><strong aria-hidden="true">2.9.</strong> Writers, Readers, batching</a></li></ol></li><li class="chapter-item expanded "><a href="ch03-00-advanced.html"><strong aria-hidden="true">3.</strong> Advanced guides</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="Advanced/Migrations.html"><strong aria-hidden="true">3.1.</strong> Migrations</a></li><li class="chapter-item expanded "><a href="Advanced/Sync.html"><strong aria-hidden="true">3.2.</strong> Sync</a></li><li class="chapter-item expanded "><a href="Advanced/CreateUpdateTracking.html"><strong aria-hidden="true">3.3.</strong> Automatic create/update tracking</a></li><li class="chapter-item expanded "><a href="Advanced/AdvancedFields.html"><strong aria-hidden="true">3.4.</strong> Advanced fields</a></li><li class="chapter-item expanded "><a href="Advanced/Flow.html"><strong aria-hidden="true">3.5.</strong> Flow</a></li><li class="chapter-item expanded "><a href="Advanced/LocalStorage.html"><strong aria-hidden="true">3.6.</strong> LocalStorage</a></li><li class="chapter-item expanded "><a href="Advanced/ProTips.html"><strong aria-hidden="true">3.7.</strong> Pro tips</a></li><li class="chapter-item expanded "><a href="Advanced/Performance.html"><strong aria-hidden="true">3.8.</strong> Performance tips</a></li><li class="chapter-item expanded "><a href="Advanced/SharingDatabaseAcrossTargets.html"><strong aria-hidden="true">3.9.</strong> iOS - Sharing database across targets</a></li></ol></li><li class="chapter-item expanded "><a href="ch04-00-deeper.html"><strong aria-hidden="true">4.</strong> Dig deeper into WatermelonDB</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="Implementation/Architecture.html"><strong aria-hidden="true">4.1.</strong> Architecture</a></li><li class="chapter-item expanded "><a href="Implementation/Adapters.html"><strong aria-hidden="true">4.2.</strong> Adapters</a></li><li class="chapter-item expanded "><a href="Implementation/SyncImpl.html"><strong aria-hidden="true">4.3.</strong> Sync implementation</a></li></ol></li><li class="chapter-item expanded "><a href="ch04-00-deeper.html"><strong aria-hidden="true">5.</strong> Other</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="Roadmap.html"><strong aria-hidden="true">5.1.</strong> Roadmap</a></li><li class="chapter-item expanded "><a href="CONTRIBUTING.html"><strong aria-hidden="true">5.2.</strong> Contributing</a></li><li class="chapter-item expanded "><a href="CHANGELOG.html"><strong aria-hidden="true">5.3.</strong> Changelog</a></li></ol></li></ol>
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                        <h1 id="schema"><a class="header" href="#schema">Schema</a></h1>
<p>When using WatermelonDB, you're dealing with <strong>Models</strong> and <strong>Collections</strong>. However, underneath Watermelon sits an <strong>underlying database</strong> (SQLite or LokiJS) which speaks a different language: <strong>tables and columns</strong>. Together, those are called a <strong>database schema</strong> and we must define it first.</p>
<h2 id="defining-a-schema"><a class="header" href="#defining-a-schema">Defining a Schema</a></h2>
<p>Say you want Models <code>Post</code>, <code>Comment</code> in your app. For each of those Models, you define a table. And for every field of a Model (e.g. name of the blog post, author of the comment) you define a column. For example:</p>
<pre><code class="language-js">// model/schema.js
import { appSchema, tableSchema } from '@nozbe/watermelondb'

export const mySchema = appSchema({
  version: 1,
  tables: [
    tableSchema({
      name: 'posts',
      columns: [
        { name: 'title', type: 'string' },
        { name: 'subtitle', type: 'string', isOptional: true },
        { name: 'body', type: 'string' },
        { name: 'is_pinned', type: 'boolean' },
      ]
    }),
    tableSchema({
      name: 'comments',
      columns: [
        { name: 'body', type: 'string' },
        { name: 'post_id', type: 'string', isIndexed: true },
      ]
    }),
  ]
})
</code></pre>
<p><strong>Note:</strong> It is database convention to use plural and snake_case names for table names. Column names are also snake_case. So <code>Post</code> become <code>posts</code> and <code>createdAt</code> becomes <code>created_at</code>.</p>
<h3 id="column-types"><a class="header" href="#column-types">Column types</a></h3>
<p>Columns have one of three types: <code>string</code>, <code>number</code>, or <code>boolean</code>.</p>
<p>Fields of those types will default to <code>''</code>, <code>0</code>, or <code>false</code> respectively, if you create a record with a missing field.</p>
<p>To allow fields to be <code>null</code>, mark the column as <code>isOptional: true</code>.</p>
<h3 id="naming-conventions"><a class="header" href="#naming-conventions">Naming conventions</a></h3>
<p>To add a relation to a table (e.g. <code>Post</code> where a <code>Comment</code> was published, or author of a comment), add a string column ending with <code>_id</code>:</p>
<pre><code class="language-js">{ name: 'post_id', type: 'string' },
{ name: 'author_id', type: 'string' },
</code></pre>
<p>Boolean columns should have names starting with <code>is_</code>:</p>
<pre><code class="language-js">{ name: 'is_pinned', type: 'boolean' }
</code></pre>
<p>Date fields should be <code>number</code> (dates are stored as Unix timestamps) and have names ending with <code>_at</code>:</p>
<pre><code class="language-js">{ name: 'last_seen_at', type: 'number', isOptional: true }
</code></pre>
<h3 id="special-columns"><a class="header" href="#special-columns">Special columns</a></h3>
<p>All tables <em>automatically</em> have a string column <code>id</code> (of <code>string</code> type) to uniquely identify records -- therefore you cannot declare a column named <code>id</code> yourself. (There are also special <code>_status</code> and <code>_changed</code> columns used for <a href="./Advanced/Sync.html">synchronization</a> - you shouldn't touch them yourself).</p>
<p>You can add special <code>created_at</code> / <code>updated_at</code> columns to enable <a href="./Advanced/CreateUpdateTracking.html">automatic create/update tracking</a>.</p>
<h3 id="modifying-schema"><a class="header" href="#modifying-schema">Modifying Schema</a></h3>
<p>Watermelon cannot automatically detect Schema changes. Therefore, whenever you change the Schema, you must increment its version number (<code>version:</code> field).</p>
<p>During early development, this is all you need to do - on app reload, this will cause the database to be cleared completely.</p>
<p>To seamlessly update the schema (without deleting user data), use <a href="./Advanced/Migrations.html">Migrations</a>.</p>
<p>⚠️ Always use Migrations if you already shipped your app.</p>
<h3 id="indexing"><a class="header" href="#indexing">Indexing</a></h3>
<p>To enable database indexing, add <code>isIndexed: true</code> to a column.</p>
<p>Indexing makes querying by a column faster, at the expense of create/update speed and database size.</p>
<p>For example, if you often query all comments belonging to a post (that is, query comments by its <code>post_id</code> column), you should mark the <code>post_id</code> column as indexed.</p>
<p>However, if you rarely query all comments by its author, indexing <code>author_id</code> is probably not worth it.</p>
<p>In general, most <code>_id</code> fields are indexed. Occasionally, <code>boolean</code> fields are worth indexing (but it's a &quot;low quality index&quot;). However, you should almost never index date (<code>_at</code>) columns or <code>string</code> columns. You definitely do not want to index long-form user text.</p>
<p>⚠️ Do not mark all columns as indexed to &quot;make Watermelon faster&quot;. Indexing has a real performance cost and should be used only when appropriate.</p>
<h2 id="advanced"><a class="header" href="#advanced">Advanced</a></h2>
<h3 id="unsafe-sql-schema"><a class="header" href="#unsafe-sql-schema">Unsafe SQL schema</a></h3>
<p>If you want to modify the SQL used to set up the SQLite database, you can pass <code>unsafeSql</code> parameter
to <code>tableSchema</code> and <code>appSchema</code>. This parameter is a function that receives SQL generated by Watermelon,
and you can return whatever you want - so you can append, prepend, replace parts of SQL, or return
your own SQL altogether. When passed to <code>tableSchema</code>, it receives SQL generated for just that table,
and when to <code>appSchema</code> - the entire schema SQL.</p>
<p>⚠️  Note that SQL generated by WatermelonDB is not considered to be a stable API, so be careful about your transforms as they can break at any time.</p>
<pre><code class="language-js">appSchema({
  ...
  tables: [
    tableSchema({
      name: 'tasks',
      columns: [...],
      unsafeSql: sql =&gt; sql.replace(/create table [^)]+\)/, '$&amp; without rowid'),
    }),
  ],
  unsafeSql: (sql, kind) =&gt; {
    // Note that this function is called not just when first setting up the database
    // Additionally, when running very large batches, all database indices may be dropped and later
    // recreated as an optimization. More kinds may be added in the future.
    switch (kind) {
      case 'setup':
        return `create blabla;${sql}`
      case 'create_indices':
      case 'drop_indices':
        return sql
      default:
        throw new Error('unexpected unsafeSql kind')
    }
  },
})
</code></pre>
<hr />
<h2 id="next-steps"><a class="header" href="#next-steps">Next steps</a></h2>
<p>➡️ After you define your schema, go ahead and <a href="./Model.html"><strong>define your Models</strong></a></p>

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